A localized surface plasmon refers to a quantized collective motion of free electrons occurring when they are confined in nanometer-sized metal particles or structures. Since the resonance wavelength at which the localized surface plasmon is excited is very sensitively dependent on the size and shape of particles and the change in refractive index of surrounding medium, studies on their applications to biochemical sensors have been extensively carried out.
Unlike a propagating surface plasmon resonance, wherein excitation occurs at the metal/dielectric interface when the condition of both momentum and energy conservation is satisfied in 2-dimensionally confined metal film, the localized surface plasmon resonance occurring in 3-dimensionally confined metallic nanostructures is characterized by a resonant light absorption and a strong scattering accompanied as a results of energy relaxation. The light absorption and scattering can be used as method of spectroscopic signal detection in a sensor application.
Although detection of signals using light absorption is relatively easy, a considerable density of nanostructures is necessary to achieve sufficient light absorption to measure the decreased intensity from a light source. In contrast, detection of signals using light scattering, commonly utilized for imaging of biomolecules and cells, is advantageous in that background noise effect caused by light source can be excluded since only the light signals scattered purely from nanostructures are detected and sensing using single particle is possible. Thus, the signal detection using light scattering facilitates multiplexed detection with high density and avoids the issue of inhomogeneous line broadening of localized surface plasmon resonance spectra resulting from the size and shape distribution.
A localized surface plasmon resonance sensor may be used to detect target molecules with a very small molecular weight and trace amounts of components compared with the surface plasmon resonance sensor whose decay length is hundreds of nanometers or longer since the distribution of local electric field allowing recognition of change in external environment is limited within several to tens of nanometers from the metal nanostructure.
The resolution of the localized surface plasmon resonance sensor is improved as the linewidth of the resonance spectrum is narrower, the intensity of the local electric field near the metal nanostructure is stronger, and the sensitivity to change in external environment represented by the change in resonance wavelength in response to the change in the refractive index of the surrounding material is higher.
In general, as the shape of the metal nanostructure changes from spherical to ellipsoidal, the surface plasmon resonance wavelength shifts toward longer wavelength, the intensity of the local electric field increases, and the sensitivity to external change is enhanced. Among the ellipsoidal structures, prolate structure is more favorable for use in a sensor than plate-shaped oblate structure.
A similar phenomenon is observed when the size of the nanostructure is increased. As the size increases, the localized surface plasmon resonance wavelength shows redshift, the intensity of the local electric field increases, and the sensitivity to external change is enhanced. However, in this case, the collective motion of free electrons in the particles becomes incoherent, resulting in multipole modes and damping of surface plasmon, which excessively increase the linewidth of resonant optical absorption or scattering curves and thus the resolution of the sensor is decreased.
Typically, the metal nanostructure is prepared as colloidal particles in solutions by chemical or electrochemical synthesis method or fabricated on a specific support substrate by a lithographic process. When compared with the solution-based synthesis wherein reaction kinetics are adjusted during particle nucleation and growth, the lithographic process is advantageous in that it is easy to realize various geometrical structures, precise control of shape and size is possible, and it is well adopted for the fabrication of array of nanoparticles and sensor integration.
FIG. 1 illustrates a conventional type of sensing platforms of metal nanostructure based on localized surface plasmon resonance.
FIGS. 1 (a) and (b) show basic sensing platforms used in fabrication of nanostructures by a lithographic process. FIG. 1 (a) shows a cylindrical plate-type nanodisc platform 110, and FIG. 1 (b) shows a cylindrical nanorod platform 120.
The plate-type nanodisc platform 110 has an oblate structure, with the electric field direction 111 of the incident light perpendicular to the axis of rotation 112. Although the lithographic process for the plate-type nanodisc is easier as compared to the nanorod type platform 120, the intensity of local electric field and sensitivity are relatively worse. In contrast, the nanorod type platform 120 exhibits superior plasmonic resonance properties in longitudinal mode when the electric field direction 121 of the incident light is parallel to the long axis 122, but the increase in aspect ratio for improving the intensity of local electric field and sensitivity results in difficulty in the lithographic process because of reduced width in the short axis. To increase the length in the long axis without decreasing the width in the short axis is undesirable since the total volume is increased inducing a line broadening. Furthermore, the increased aspect ratio results in the reduction in cross-section of end surface of nanorod where the local electric field enhancement due to the excitation of longitudinal mode of surface plasmon is concentrated, leading to unfavorable environment for detection of thin analytes positioned on that surfaces. A similar problem occurs also in the oblate-type nanodisc platform 110. In the plate-type nanodisc platform 110, the intensity of local electric field and sensitivity are increased when the electric field direction 111 of the incident light is perpendicular to the axis of rotation 112 and as the thickness-to-area ratio of the disc increases. However the local electric field is concentrated at the edge of the disc rather than the surface region where the major portion of analytes are positioned on through surface functionalization, deteriorating the sensing resolution.
Also, the plate-type nanodisc platform 110 and the nanorod type platform 120 have problems when light scattering is used for single particle detection. Typically, the spectroscopic scattering properties are analyzed using a dark-field microscope or a total internal reflection microscope. Referring to FIG. 1 (a), in dark field microscopy, light 113 is incident on the surface of a sample to be analyzed with an incidence angle θi from a direction 112 normal to the surface, and only the diffuse scattering components excluding the specular components are detected. The incidence angle θi makes the electric field be divided into two components: one is perpendicular and the other is parallel to the sample surface, diminishing the portion of component for optimized localized surface plasmon.
In analysis using total internal reflection microscopy, both S and P waves can be used. And, since the evanescent electric field formed at the interface of total reflection is either perpendicular or parallel to the sample, the problem of electric field division which occurs in the dark field microscopy can be avoided.